In many systems, such as control systems which use computers or other appropriate control logic, it is desirable to supply such control logic with input signals of various kinds for controlling various operations or for processing input data signals. In many applications it is desirable that the utilization device, i.e., the control logic circuitry, be electrically isolated from the source of the input signals which are being supplied thereto. One type of device which provides such isolation is an optical isolator wherein the input signal is converted to a light signal, as by a light-emitting diode (LED), which light light-emitting signal is thereupon received by a photoresponsive device, such as a high speed photo-diode transistor pair. Transmission of the input signal in the form of a light signal provides appropriate electrical isolation between the input circuitry which supplies the LED and the output circuitry which is supplied from the photoresponsive device to the control logic.
Presently available optical isolator circuits are usually designed for very specific applications so as to accept input signals having particular characteristics, the design thereby being tailored to such characteristics. For example, certain isolators may be designed for accepting only AC signals, or for accepting only DC signals of a specified polarity and, in general, such design is usually responsive only to signals of a particular relatively narrow range of amplitude levels. Accordingly, such optical isolators are not generally available for widespread use in many different applications which may involve the acceptance of input signals of many different characteristics, i.e., AC signals or DC signals of either polarity, and signals having a wide dynamic range of amplitudes.
Even where such optical isolator circuitry is specifically designed for a particular use, such circuitry is subject to picking up undesirable noise signals of varying amplitudes which affect the output signal being supplied to the utilization device. A particular problem arises, for example, when extremely high amplitude noise signals are picked up by the isolator circuitry. For example, in machine tool control applications, the starting and stopping of motors which are used in the machine tool system produces extremely high amplitude oscillatory transient signls, commonly called "showering arc", sometimes as high as several thousand volts. The envelope of these signals tends to be several hundred microseconds long. Such transient signals are picked up by the input signal lines of the isolator from cables located in the general vicinity thereof. These transient noise signals normally have extremely wide bandwidths and the ability to filter them by conventional filter means is extremely difficult.
Moreover, even were the signals somehow to be filtered before being applied to the control logic circuitry at the output of the isolator device, the signals would still be present at the input circuitry to the LED. In such circumstances, while the overall system may be designed to achieve a relatively high degree of noise immunity (i.e., the output signal from the device has reduced noise levels), the input circuitry including the LED device will still be directly subject to the transient signal and the electric components used therein may be destroyed by the high amplitude transient signals. Hence, the circuitry, though having some noise immunity, often has low noise survivability to the application of such transients because the latter levels are so high that damage or destruction to the elements thereof occurs.
For example, certain present day isolator circuitry uses an LED and resistor which has a Zener diode connected in parallel therewith so as to provide a substantially constant voltage across the LED and resistor and, therefore, a substantially constant current through the LED. However, the Zener diode, apart from being costly, is subject to destruction by high amplitude transients of any polarity. Moreover, transients of negative polarity, even having relatively low amplitudes, can cause the Zener diode to be effectively destroyed by the occurrence thereof. Insofar as is known at present, no optical isolator circuitry is available which can adequately survive high transient noise signals which may occur in many applications nor do such presently available optical isolator circuits permit the acceptance of input signals over wide ranges of input amplitudes for both AC signals and DC signals of either polarity.